The study will assess initially amdoxovir (300 mg or 500 mg twice daily) or tenofovir DF 300 mg once daily, both in combination zidovudine 300 mg twice daily plus failing third drug, but then with lopinavir/ritonavir (400 mg/100 mg twice daily) after Week 2. Subjects who received amdoxovir (300 mg or 500 mg twice daily) and benefited from the drug may choose to enroll in the 36-week open-label study.